docs: def defineGeometry(self):
          """Set the 3D geometry of the cell."""
          self.soma.L = 18.8
          self.soma.diam = 18.8
          self.soma.Ra = 123.0
  
          self.dend.L = 200.0
          self.dend.diam = 1.0
          self.dend.Ra = 100.0